High-voltage multiple core transformer



June 10, 1952 Q; A KERNS 2,600,057

HIGH-VOLTAGE MULTIPLE CORE TRANSFORMER FiledMay 18, 1949 l. l. u r

ATTORNEY.

Patented June 10, 1952 HIGH-VOLTAGE MULTIPLE CORE TRANSFORMER Quentin A. Kerns, Berkeley, Ualif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application May 18, 1949, Serial No. 94,047

(Cl. F75-356) 4 Claims.

This invention relates to electrical transformers, especially pulse transformers and, in particular, to transformers for the production of pulses of extremely high voltage and short durationby means of the additive flux effect of a multiplicity of separately magnetized cores.

Heretofore, the voltage transformation between a primary and a secondary winding of a conventional transformer, such as a power transformer or the like, was due only to the ratio of the number of turns in the primary winding to to the'number of turns in the secondary winding. For relatively low voltages of the order of about 10,000 volts, such conventional transformer provides satisfactory voltage transformation, However, for high voltage, high frequency pulses of the order of 100,000 volts or more and of about 0.1 microsecond duration, the insulation and the stray capacitance of the windings become major limiting factors in providing a satisfactory transformer construction. In conventional transformers high voltage on closely spaced windings tends to break down the insulation by arcing between windings, between winding and the transformer cores, and between close or adjacent turns of the same winding. Also,v in conventional transformers, the stray capacitance of transformer components limits the transformation of pulses of nonsinusoidal waveshape such as square waves and sawtooth waves in non capacitive circuits. Such stray capacitance exists between winding turns, between windings, and between windings and the cores. The present invention, by a unique winding and core arrangement, overcomes the above objections and permits the manufacture of a high voltage pulse transformer capable of producing and withstanding voltage pulses of the order of 75,000 volts in air and about 750,000 volts in oil.

Accordingly, it is an object of the present invention to provide a multiple core pulse transformer of low stray capacitance capable of producing and withstanding extremely high voltage pulses.

AAnother object is to provide a multiple core pulse transformer of low stray capacitance and low leakage reactance capable of producing and withstanding extremely high voltage pulses.

'Still another object is to provide a multiple core pulse transformer with each of the cores having individually wound primary windings and all of the primary windings connected in parallel.

A further object is to provide a multiple core transformer in which the ratio of transformation is proportional to the number of cores.

Still a further object is to provide an easily constructed pulse transformer with readily accessible windings.

Other objects and advantages of the invention will become more apparent when considered in conjunction with the following description and drawings in which:

Figure l is a schematic view of the elements of a transformer embodying the invention;

Fig. 2 is an elevation of a transformer utilizing a single secondary winding;

Fig. 3 is a sectional elevation of a transformer utilizing a double secondary winding; and

Fig. 4 is a perspective view of a transformer partly cut away employing the embodiment illustrated in Fig. 3 and disclosing mechanical details of the windings.

In general, my invention comprises a transformer having a plurality of cores insulated from each other and with each core having a separate primary winding. The leads of the primary windings preferably are all connected in parallel, permitting the magnetic flux of the primary winding to induce a cumulative voltage on a secondary winding which successively encompasses all of the cores in series.

Attention is now directed to Fig. 1 illustrating schematically the relation of the several transformer elements for carrying out the transformer operation. A plurality of ferromagnetic transformer cores, as for example, cores II, I2, and I3, formed preferably of magnetic material of low hysteresis and eddy current loss have corresponding apertures I4, I5, and I6. A primary winding I8 consisting of a single turn of conductive material such as copper ribbon or wire through the aperture I4 of core II is connected in parallel with identical turns I9- and 20 through cores I2 and I3. A source of variable current capable of supplying voltage pulses is applied to parallel windings 25. A secondary winding 22 of conductive material, such as copper wire, is threaded through the apertures I4, I5, and I6 of cores II, I2, and I3 in a series manner such that the voltage induced in the secondary winding 22 by magnetic flux in cores II, I2, and I3 will be additive, Primary windings I8, I9, and 20 are connected in parallel so as to allow each of the windings, Iwhen energized by a varying current, to produce the same amount of magnetic flux in each core. Since secondary winding `22 passes through all the cores in series and consequently has the flux of each core impressed on a portion of its winding, the voltages induced therein will be the sum of the voltages induced by each individual core. It will be noted then that even though the ratio of transformation of each core is only unity, the cumulative effect of the magnetic ux oi the three cores Il, l2, and I3 upon the secondary 22 results in a voltage output three times as great as that of the primary voltage. The above procedure may be carried out using any number of cores, and the voltage induced in the secondary winding will be dependent not only on the ratio of the number of turns in the primary and secondary windings, but also upon the number of cores used. The ratio of transformation then becomes C where C is the number of cores, Np is the number of turns around each core of the primary winding, and Ns is the number of turns of the secondary through all the cores.

If now, cores Il, l2, and i3 are placed one over the other so that apertures i4, l5, and i6 coincide vertically, secondary 22 may preferably take the form of a straight conductor, resulting in a transformer having a construction as shown in Fig. 2. The cores 23 of Fig. 2 are stacked and aligned vertically so as to form a hollow cylinder with a. core passageway 26. The cores are electrically insulated from each other by means of suitable insulating material 24, such as polystyrene, in order to prevent any induced eddy currents from owing between the individual cores. The thickness ofthe insulation is suincient to enable the paralleled single turn primaries 25 to pass between the cores and then around their respective coresy without touching either the adjacent turns or the core itself and normally a suitable insulating covering is placed upon the primary turn prior to assembly of the structure. In order to maintain minimum insulation thickness, the primary winding 25 preferably takes the form of strips of copper ribbon which are easily bent and formed. To provide for the removal of accumulated electrostatic charges on the insulated cores, each core is connected to a ground 31. The secondary winding 22 which preferably takes the form of a copper rod, makes a single turn through the center of the core passageway 26 of all the cores, and the output voltage induced therein is obtained between a pair of terminals 2l of secondary 22. If the voltage output of secondary winding 22 is desired relative to ground, one side of winding 22 may be grounded as shown. rlhe transformer inherently possesses high resistance to Voltage breakdown between secondary winding 22 and the cores 23, and betweenl secondary 22 and primary 25, because the physical location of the secondary 22 is such that it need not touch any other part of the transformer. The value of the breakdown potential may be still further increased by submerging the entire transformer in a fluid of high electrical` resistance, as for example, transformer oil.

Fig. 3 illustrates a preferred embodiment oi the present invention, utilizing secondary winding 22 consisting of two conductors 21 and 23 which are connected so as to yield double the voltage output provided by the embodiment shown in 2. One end of conductor 21 is connected to the inside of a bottom core 2S of the pile of grounded cores 23 at a point 3D and inclined so that it emerges in the center of the aperture of a top core 3l. In a similar manner, one endof conductor 28 enters the center of CII the area of the aperture of the bottom core 29 and is inclined parallel to conductor 21 along the core passageway 2B and is grounded to the top of the inside of core 3l at a point 32. Connections to the cores such as grounding connections at points y30 and 32 are preferably made by welding or soldering, so that mechanical rigidity is imparted by the connections as Well as electrical conductivity. The utilization of two conductors 21 and 28 does not reduce the breakdown voltage between the secondary windings 22 and the cores 23 or between the sec- OIldary windings 22 and the primary windings 25, since, as will be shown, the voltage tension relationships of the embodiment of Fig. 2 are unchanged. If, now, we consider the bottom core 29 of the core pile 23, the voltage diiferential between conductor 23 of secondary 22 and the grounded core 29 is at a maximum. Conductor 28, however, is in the center of the aperture area of core 29, so that insulation strength between secondary 22 and cores 23 is also at a maximum. Conductor 21 is grounded at the bottom core 28 at point 38 and, consequently, has no voltage differential with respect to the core 23. As we move up conductor 28 through the core passageway 26, the distance between conductor 28 and the cores 23 decreases but likewise and in the same proportion the voltage diiference decreases, becoming zero at the top of core 3l at point 32. In a similar manner, conductor 21 has an increasing voltage induced in it as we consider it from the bottom to the top of the core pile 23, but proportional to the voltage increase, conductor 21 also inclines toward the center of the core passageway 26 until at the upper edge it is at the center of the area of the aperture of the top of core 3| where it has maximum insulation strength. Thus, the potential necessary for voltage breakdown of the transformer windings remains constant between the cores 23 and the conductors 21 and 28 of secondary 22 along the entire length of the core passageway 26. Similarly, the voltage gradient between the conductors 21 and 28 is constant along the length of core passageway 2S, since the secondaisT windings are constructed parallel to each other. The voltages induced in conductors 21 and 28 are of equal magnitude and of opposite phase; hence, the resultant voltage between conductors 21 and 28 is equal to twice the voltage of either conductor to a neutral ground rod 33.

Capacitance of the windings is a function of the proximity of the turns of the windings as well as the number of turns. In one form of my invention, the primary winding as illustrated consists of only a single turn about each core with each turn physically separated by a grounded core from an adjacent turn. It is to be understood, however, that a plurality of turns of primary winding may be made about each core without departing from the larger aspects of the invention. When only one turn of primary winding per core is used, however, the input capacitance of the primary is diminished for all practical purposes to a negligible value of the order of micro-micro farads. Likewise, the capacitance of the secondary winding has also been reduced to a negligible value of the order of 50 micro -micro farads when utilizing only one or two secondary turns about the cores 23. The large spacing between secondary rods 21 and 28 and between the secondary rods and the primary windings is also an important factor in the reduction of the amount of stray capacitance. The leakage reactance of my transformer is likewise held to a minimum value, since the transformer structure allows nearly all of the iiux of the primary windings to link with that of the secondary windings, providing a transformer of high efficiency.

Fig. 4 is a perspective view of the transformer embodiment illustrated in Fig. 3 and discloses in detail several structural features of the preferred form of the pulse transformer. As an example, in one satisfactory form, the core structure 23 is about high and made up of fifteen layers of a ferromagnetic material comprising iron, nickel, and molybdenum. Each layer consists of a wide ribbon of the magnetic material wound 1 1/2 thick upon a square form having a 3 side. The resulting core layer has the form of a square having a 6 side and a 3 square aperture. Each core layer is separated from adjacent core layers by 1A of polystyrene insulation 24. The primary winding consists of fifteen strips of 3A wide copper tape, each strip passing through the insulation 24 and making a single turn around a corresponding core as shown. All of the turns of the copper strips are connected in parallel by soldering them to a pair of copper straps 4l and 42. In order that the cores 23 do not short circuit the primary windings, an insulating covering of polystyrene is wound around the separate primary turns where they touch the cores.

The secondary winding 22 is made of two M3 diameter copper rods 21 and 28 so as to possess mechanical rigidity and are brazed or soldered to the top and bottom of cores 23 at points 32 and 30. Each core is electrically connected to a copper grounding elbow 36 by brazing and all of the elbows are connected to a grounded copper strip 31, to which grounding rod 33 is also connected.

The above embodiment has a step-up ratio of about 15 to 1 with a purely resistive load and about 25 to 1 when connected to a highly capacitive load. For example, with a primary input consisting of a 20,000 volt pulse having a 0.15 microsecond duration and having a pulse recurrence frequency of about 100 times per second, an output of 300,000 to 500,000 volts, depending on the type of load, may be obtained. The transformer is capable of withstanding about 75,000

volts in air and about 750,000 volts when entirely submerged in transformer oil.

It will be understood that the invention is capable of many refinements which will readily occur to those skilled in the art; I intend, therefore, to be limited only as indicated by the scope of the following claims, wherein I claim:

1. A high voltage pulse transformer comprising a plurality of toroidal magnetic cores disposed in superposed and spaced relation, a core primary winding surrounding a radial cross section of each core, a plurality of spaced insulating rings disposed in the spaces between the respective cores, a conductor interconnecting said cores, a pair of spaced conductors interconnecting the corresponding terminals of each of said core primary windings in parallel, and a secondary winding extending axially through the generally cylindrical space defined by said corering assembly, thence as a single conductor bight once about the end portion of each terminal core of said assembly to external electrical terminals.

2. A high voltage pulse transformer comprising a plurality of toroidal magnetic cores disposed in superposed and spaced relation, a core primary winding consisting of a single conducting loop passing once only around a radial cross section of each core, a plurality of spaced insulating rings disposed in the spaces between the respective cores and constituting a core-ring assembly, a conductor interconnecting said cores, a pair of spaced conductors interconnecting the corresponding terminals of each of said loops in parallel and a single conductor secondary winding extending axially and once only through the generally cylindrical space defined by said core-ring assembly, thence as a bight once only about the end portion of each terminal core of said assembly to external electrical terminals.

3. A high voltage pulse transformer comprising a plurality of toroidal magnetic cores disposed in superposed and spaced relation, a core primary winding surrounding a radial cross section of each core, a plurality of spaced insulating rings disposed in the spaces between the respective cores, a conductor interconnecting said cores, a pair of spaced conductors interconnecting the corresponding terminals of each of said core primary windings in parallel, a secondary winding having a pair of conductors connected to opposite terminal cores respectively of said core-ring assembly and extending outwardly in opposite directions through the center of said terminal cores, thence as a bight about the end portion of said terminal cores to external electrical terminals.

4. A high voltage pulse transformer comprising a plurality of toroidal magnetic cores disposed in superposed and spaced relation, a core primary winding consisting of a single conducting loop passing once only around a radial cross section of each core, a plurality of spaced insulating rings disposed in the spaces between the respective cores and constituting a core-ring assembly, a conductor interconnecting said cores, a pair of spaced conductors interconnecting the corresponding terminals of each of said loops in parallel, a secondary winding having a pair of conductors connected to opposite terminal cores, respectively of said core-ring assembly and extending outwardly in opposite directions through the center of said terminal cores, thence as a bight about the end portion of said terminal cores to electrical terminals.

QUENTIN A. KERNS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,504,611 Dorfman Aug. 12, 1924 1,831,886 Ross Nov. 17, 1931 2,251,373 Olsson Aug. 5, 1941 2,531,820 Lindenblad Nov. 28, 1950 FOREIGN PATENTS Number Country Date 12,264 Great Britain A. D. 1886 460,449 Great Britain Jan. 28, 1937 

